The Cling MediaRenderer is a Java implementation of a UPnP MediaRenderer device. You can turn any device into a network controllable media player, which can play local files or streams from HTTP (media) servers. It even handles multiple streams simultaneously on a single host.
Screenshots are available here.
The playback backend is currently based on the gstreamer project. If you are using Linux, you should be able to install gstreamer as usual or you have it probably already installed. On Windows, use this installer package. For OS X you can either build it yourself with Macports or you can try this installer.
Note: Specify the location of your gstreamer install as a system property (e.g. on Linux):
java -Djna.library.path=/usr/lib -jar cling-mediarenderer.jar
If you want to run the MediaRenderer's display in windowed mode (not fullscreen), start it with the -w argument.